Vanessa Redgrave, who has narrated three series of Call the Midwife, is to appear on screen in the BBC drama for the first time.

The actor plays Jennifer Worth, the writer whose real-life stories formed the basis of the show. Jessica Raine has bowed out after three series as a younger Worth.

Redgrave is to join the cast for a Christmas episode, and will continue as narrator. Producers also announced new cast members for a fourth series, including Linda Bassett, known for appearances in East Is East and Lark Rise To Candleford, and Charlotte Ritchie, who has featured in Fresh Meat.

The eight-episode series will follow the community of nuns and nurses at Nonnatus House and the people of Poplar at the dawn of the 1960s. Filming has begun at locations including Chatham dockyard and Chertsey in Surrey.

The show's writer and executive producer, Heidi Thomas, said: "The medical, social, and emotional stories at the heart of Call the Midwife continue to inspire and compel me. The Nonnatus House nurses and midwives feel like my best-loved friends."

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